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1. Stochastic FDTD Modeling of Propagation Loss due to Random Surface Roughness in Sidewalls of Optical Interconnects

   https://doi.org/10.23919/USNC-URSINRSM51531.2021.9336433  

   Guiana, Brian; Zadehgol, Ata (February 2021, 2021 United States National Committee of URSI National Radio Science Meeting (USNC-URSI NRSM))

   null (Ed.)

   The dielectric waveguide (WG) is an important building block of high-speed and high-bandwidth optical and opto-electronic interconnect networks that operate in the THz frequency regime. At the interface of Si/SiO 2 dielectric waveguides with width above w = 2.5 μm and anisotropic surface roughness, transverse electric (TE) mode surface wave propagation can experience a loss of approximately a = 2 dB/cm; however, propagation losses increase rapidly to near a = 44 dB/cm as the width decreases to w = 500 nm, due to increased interaction of surface waves and sidewall surface roughness that exhibits random distribution inherent to the manufacturing process. Previous works have developed analytic expressions for computing propagation loss in a single dielectric waveguide exhibiting random roughness. More recent works report a = 0.4 dB/cm noting the non-trivial estimation errors in previous theoretical formulations which relied on planar approximations, and highlight the discrepancy in planar approximations vs. the 3-D Volume Current Method. A challenge that remains in the path of designing nanoscale optical interconnects is the dearth of efficient 3-D stochastic computational electromagnetic (CEM) models for multiple tightly coupled optical dielectric waveguides that characterize propagation loss due to random surface roughness in waveguide sidewalls. Through a series of theoretical and numerical experiments developed in the method of finite-difference time-domain (FDTD), we aim to develop stochastic CEM models to quantify propagation loss and facilitate signal & power integrity modeling & simulation of arbitrary configurations of multiple tightly-coupled waveguides, and to gain further insights into loss mechanisms due to random surface roughness in optical interconnects. 

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2. A Non-Reciprocal Surface Acoustic Wave Filter Based on Asymmetrical Delay Lines and Parametric Interactions

   https://doi.org/10.1109/MEMS46641.2020.9056343  

   Bahamonde, Jose A.; Kymissis, Ioannis (January 2020, 2020 IEEE 33rd International Conference on Micro Electro Mechanical Systems (MEMS))

   null (Ed.)

   Acoustic devices have played a major role in telecommunications for decades as the leading technology for filtering in RF and microwave frequencies. While filter requirements for insertion loss and bandwidth become more stringent, more functionality is desired for many applications to improve overall system level performance. For instance, a filter with non-reciprocal transmission can minimize losses due to mismatch and protect the source from reflections while also performing its filtering duties. A device such as this one was originally researched by scientists decades ago. These devices were based on the acoustoelectric effect where surface acoustic waves (SAW) traveling in the same direction are as drift carriers in a nearby semiconductor are amplified. While several experiments were successfully demonstrated in [1], [2], [3]. these devices suffered from extremely high operating electric fields and noise figure [4], [5]. In the past few years, new techniques have been developed for implementing non-reciprocal devices such as isolators and circulators without utilizing magnetic materials [6], [7], [8], [9]. The most popular technique has been spatio-temporal modulation (STM) where commutated clock signals synchronized with delay elements result in non-reciprocal transmission through the network. STM has also been adapted by researchers to create non-reciprocal filters. The work in [10] utilizes 4 clocks signals to obtain a non-reciprocal filter with an insertion loss of -6.6 dB an isolation of 25.4 dB. Another filter demonstrated in [11] utilizes 6 synchronized clock signals to obtain a non-reciprocal filter with an insertion loss of -5.6 dB and an Isolation of 20 dB. In this work, a novel non-reciprocal topology is explored with the use of only one modulation signal. The design is based on asymmetrical SAW delay lines with a parametric amplifier. The device can operate in two different modes: phase coherent mode and phase incoherent mode. In phase coherent mode, the device is capable of over +12 dB of gain and 20.2 dB of isolation. A unique feature of this mode is that the phase of the pump signal can be utilized to tune the frequency response of the filter. Under the phase-incoherent mode, the pump frequency remains constant and the device behaves as a normal filter with non-reciprocal transmission exhibiting over +7 dB of gain and 17.33 dB of isolation. While the tuning capability is lost in this mode, phase-coherence is no longer necessary so the device can be utilized in most filtering applications. 

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3. Structure–Property Relationships in High-Rate Anode Materials Based on Niobium Tungsten Oxide Shear Structures

   https://doi.org/10.1021/acsaem.2c03573  

   Salzer, Luke D.; Diamond, Brian; Nieto, Kelly; Evans, R. Colby; Prieto, Amy L.; Sambur, Justin B. (January 2023, ACS Applied Energy Materials)

   Nb16W5O55 emerged as a high-rate anode material for Li-ion batteries in 2018 [Griffith et al., Nature2018, 559 (7715), 556−563]. This exciting discovery ignited research in Wadsley−Roth (W−R) compounds, but systematic experimental studies have not focused on how to tune material chemistry and structure to achieve desirable properties for energy storage applications. In this work, we systematically investigate how structure and composition influences capacity, Li-ion diffusivity, charge−discharge profiles, and capacity loss in a series of niobium tungsten oxide W−R compounds: (3 × 4)-Nb12WO33, (4 × 4)-Nb14W3O44, and (4 × 5)-Nb16W5O55. Potentiostatic intermittent titration (PITT) data confirmed that Li-ion diffusivity increases with block size, which can be attributed to an increasing number of tunnels for Li-ion diffusion. The small (3 × 4)-Nb12WO33 block size compound with preferential W ordering on tetrahedral sites exhibits single electron redox and, therefore, the smallest measured capacity despite having the largest theoretical capacity. This observation signals that introducing cation disorder (W occupancy at the octahedral sites in the block center) is a viable strategy to assess multi-electron redox behavior in (3 × 4) Nb12WO33. The asymmetric block size compounds [i.e., (3 × 4) and (4 × 5) blocks] exhibit the greatest capacity loss after the first cycle, possibly due to Li-ion trapping at a unique low energy pocket site along the shear plane. Finally, the slope of the charge−discharge profile increases with increasing block size, likely because the total number of energy-equivalent Li-ion binding sites also increases. This unfavorable characteristic prohibits the large block sizes from delivering constant power at a fixed C-rate more so than the smaller block sizes. Based on these findings, we discuss design principles for Li-ion insertion hosts made from W−R materials. 

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4. Indoor Wireless Channel Properties at Millimeter Wave and Sub-Terahertz Frequencies

   https://doi.org/10.1109/globecom38437.2019.9013236  

   Xing, Yunchou; Kanhere, Ojas; Ju, Shihao; Rappaport, Theodore S. (May 2019, 2019 International Conference on Communications)

   null (Ed.)

   This paper provides indoor reflection, scattering, transmission, and large-scale path loss measurements and models, which describe the main propagation mechanisms at millimeter wave and Terahertz frequencies. Channel properties for common building materials (drywall and clear glass) are carefully studied at 28, 73, and 140 GHz using a wideband sliding correlation based channel sounder system with rotatable narrow-beam horn antennas. Reflection coefficient is shown to linearly increase as the incident angle increases, and lower reflection loss (e.g., stronger reflections) are observed as frequencies increase for a given incident angle. Although backscatter from drywall is present at 28, 73, and 140 GHz, smooth surfaces (like drywall) are shown to be modeled as a simple reflected surface, since the scattered power is 20 dB or more below the reflected power over the measured range of frequency and angles. Partition loss tends to increase with frequency, but the amount of loss is material dependent. Both clear glass and drywall are shown to induce a depolarizing effect, which becomes more prominent as frequency increases. Indoor propagation measurements and large-scale indoor path loss models at 140 GHz are provided, revealing similar path loss exponent and shadow fading as observed at 28 and 73 GHz. The measurements and models in this paper can be used for future wireless system design and other applications within buildings for frequencies above 100 GHz 

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5. A Novel Low Loss 3D System-in-Package Approach for 60GHz Antenna on Chip Applications

   Adamshick, Stephen; Govindarajulu, Sandhiya; Alwan, Elias. (August 2020, 2020 IEEE 63rd International Midwest Symposium on Circuits and Systems (MWSCAS))

   null (Ed.)

   This paper presents a novel low loss 3D system in package (SiP) approach for achieving antenna-on-chip integration. Specifically, this design uses 3D through silicon via (TSV) technology to achieve a vertical SiP phased array radio. The fully integrated package consists of a digital baseband chip, a radio frequency integrated circuit (RFIC), and lastly a microstrip patch phased array. The 3D TSVs achieve an insertion loss of less than 0.4 dB/pair at millimeter-wave frequencies. The differential fed microstrip patch array achieves a return loss of 40 dB at a 60 GHz center frequency with 4 GHz instantaneous bandwidth. The antenna array achieves an E and H plane realized gain of 17.1 dBi for a 4×4 element design. In addition, this design approach enables individual fabrication of each element to maximize yield with low cost assembly using ball grid array (BGA) technology. Lastly, this design does not require special design rules that comprise either transistor or antenna performance as compared to other methods outlined in antenna on chip design. 

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